OYSTERSHELL SCALE

Oystershell scale is a serious ornamental pest of southern Alberta. It attacks over 120 species of ornamental trees and shrubs, most commonly cotoneaster hedges, apple, crabapples, hawthorn, mountain ash, green ash, and many others. Without proper recognition, this pest will destroy countless plant materials in Alberta.

Infested cotoneaster hedges die-back in patches, leaving large voids in the hedge. Close inspection of the twigs reveals thousands of adult oyster-shaped insects encrusting the stems. Natural predator insects are not sufficient to control this pest.

The treatment window for oystershell scale is very narrow, normally a two-week window in June while the “crawler” stage of the insect is active. Crawlers are highly susceptible to insecticidal sprays during this time.  Commonly used dormant oil sprays are largely ineffective during the dormant season because the eggs are protected beneath the waxy dead adult shell.  Infested trees can be sprayed in the same manner as hedges.  Conventional insecticides are most effective.

Very heavily infested (>50% dead) cotoneaster hedges are unsightly and will never recover as well as homeowners would like. In this case, the best thing to do is:

1) Cut the hedge back to about 8″ above grade in late winter, before bud-break. You may get away with cutting back sections of the hedge only;

2) In June, spray the sprouting stumps with a contact insecticide (oystershell will attack stems right to the base of the plant);

3) As your new hedge grows, monitor pest populations and spray in June if required. An annual spray is the best way to keep infested plants looking good.

To book pesticide applications, please call early in the season, as spray timing is very important, and availability is limited.